1. Field of the Invention
The present invention relates generally to fluid flow control devices and, more particularly, to a uniquely configured booster valve for integration into a pneumatic circuit of a valve actuation system.
2. Description of the Related Art
Pneumatic valve actuation systems typically comprise a source of compressed air that is routed through a network of fluid conduits such as pipes. The compressed air is typically provided by an air compressor. The compressed air is routed to a positioner that ultimately controls the flow of compressed air into and out of an actuator. More particularly, the positioner provides a metered flow of compressed air into alternate ends of the actuator in response to a positioner input signal. The actuator may be, for example, a double acting actuator comprising a reciprocating piston sealed within a cylinder. The cylinder of the double acting actuator has a working chamber on each end, with the piston being slidably captured between such chambers. The chambers of the actuator simultaneously receive and exhaust the compressed air as the piston moves back and forth within the cylinder. The piston typically has a shaft which extends from one end of the cylinder, with the shaft being connected to a valve or other component to be moved or actuated in a prescribed manner.
The actuation system moves or strokes the piston by forcing air into a first end of the cylinder while simultaneously exhausting air out of a second end of the cylinder in order to advance the piston in a first direction along the axis of the cylinder. Conversely, the actuation system may also force air into the second end of the cylinder while simultaneously exhausting air out of the first end of the cylinder in order to move the piston along the axis of the cylinder in a second direction opposite the first direction. By driving the air into alternate ends of the cylinder, the piston is moved such that the shaft can be displaced in any position for doing useful work. Actuation systems are commonly used in large scale applications such as in power plants and refineries for controlling system components such as a working valve. In such applications, it may be desirable to repeatedly position the piston to within thousandths of an inch within a very short stroking time. In addition, large scale applications may utilize large volume actuators to react to the high forces that are typical of severe service control valves.
When a large volume actuator is utilized in the pneumatic circuit of the actuation system, the positioner, acting alone, may be unable to supply and exhaust a sufficient volume of compressed air to the actuator within a given time period. As a result, such pneumatic circuits having large volume actuators may be incapable of achieving a quick stroking speed of the piston. In these cases, a known practice in the prior art is to install first and second boosters between the positioner and respective first and second ends of the actuator. When the boosters are integrated into the pneumatic circuit of the actuation system, the positioner typically facilitates the activation of such first and second boosters by providing pneumatic signals in the form of compressed air which is routed thereto. The boosters allow the actuation system to achieve very short stroking times by increasing the flow rate of the positioner to the first end of the cylinder while simultaneously exhausting the second end of the cylinder through a large outlet, or vice-versa. The Cv of the boosters in the exhaust mode is typically greater than the Cv in the supply mode since the exhaust capacity in the pneumatic circuit is typically the controlling factor in determining the stroking time of the piston. The boosters are each connected to the positioner through the use of signal lines, and each receive pneumatic signals via such signal lines, such pneumatic signals being operative to selectively open and close the boosters as needed to regulate the flow of the compressed air into and out of the cylinder in a prescribed manner.
In addition to being connected to the positioner through the use of the signal lines, the boosters are also each connected to the air source and to the actuator through the use larger diameter feed lines. The signal lines are typically of a smaller diameter than the feed lines, some of which supply and exhaust compressed air into and out of the cylinder. When the positioner provides a greater flow of compressed air into the signal lines, such increased pressure or “signal” is sufficient to trigger the boosters such that they are energized. When energized, the boosters allow compressed air to flow from the larger diameter feed lines into and out of the cylinder at a higher flow rate, thereby reducing the stroking time of the piston. As a result, actuator systems including the aforementioned boosters allow the actuator to achieve a relatively fast stroking time if the positioner is capable of providing a flow rate that is high enough to energize the boosters. However, where a low flow rate positioner is utilized, pneumatic circuits operating with large volume actuators may not be able to energize the boosters. Consequently, they suffer the drawback of a slow stroking speed.
However, the benefits to the actuation system that are provided by the addition of the first and second boosters are often accompanied by a performance penalty in the pneumatic circuit of the actuation system. More particularly, the increased number of active components in the pneumatic circuit often gives rise to dynamic instability wherein the piston is difficult to precisely and rapidly position. In this regard, as a result of the increased number of active components attributable to the addition of the boosters, the total requirement of compressed air out of the positioner that is needed in order to effectuate a given piston movement is increased in comparison to pneumatic circuits having a lesser number of active components. Due to the inherently compressible nature of air, the piston may not start to move toward the desired position until the pair of boosters have sufficiently pressurized. Thus, there may be an undesirable lag time between the time that the positioner receives the piston position signal and the time that the piston arrives at the desired position. Also, the piston may overshoot the final position, with overshooting occurring when the piston, moving at a relatively high rate of speed, fails to slow down as it nears the final position such that it moves past the desired position and then must reverse directions. The overshooting of the piston therefore increases the overall lag time of the actuator.
Many of the aforementioned performance penalties in existing actuation systems are attributable to the structural and functional attributes of the boosters integrated into the pneumatic circuit of the actuation system. The present invention alleviates or eliminates such performance penalties by providing a uniquely configured booster valve, a pair of which may be integrated into the pneumatic circuit of an actuation system in the above-described manner. These, and other features and advantages of the present invention, will be described in more detail below.